Ah, Christmas, the season of peace, goodwill and overindulgence. If this year is like others, I’ll probably be taking up residence on the couch after a big lunch, continuing to munch my way through packets of unhealthy snacks, and promising myself that I’ll live a more virtuous life once the New Year begins.

It was on one such occasion that I had an epiphany in the psychology of everyday life. I’d just finished the last crisp of a large packet, and the thought occurred to me that I don’t actually like crisps that much. But there I was, covered in crumbs and post-binge guilt, saturated fats coursing through my body looking for nice arteries to settle down on. In an effort to distract myself from the urge to reach for another packet, I started to think about the peculiar psychology of the situation.

Every bite seemed essential, but in a way that seem to suggest I was craving them rather than liking them. Fortunately for my confusion (and my arteries), there's some solid neuroscience to explain how we can want something we don't like.

Normally wanting and liking are tightly bound together. We want things we like and we like the things we want. But experiments by the University of Michigan's Kent Berridge and colleagues show that this isn't always the case. Wanting and liking are based on separate brain circuits and can be controlled independently.

To demonstrate this, Berridge used a method called "taste reactivity", in effect, recording the faces pulled when animals are given different kinds of food. Give an adult human something sweet and they'll lick their lips. This might sound obvious, but when you take it to the next level in terms of detail and rigour you start to get a powerful system for telling how much an animal likes a particular type of food. Taste reactivity involves defining the reactions precisely – for example, lip-licking would be defined as "a mild rhythmic smacking, slight protrusions of the tongue, a relaxed expression accompanied sometimes by a slight upturn of the corners of the mouth" – and then looking for this same expression in other species. A baby human can't tell you they like the taste like an adult can, but you can see the same expression. A chimpanzee will do the same with a sweet taste. A rat won't do exactly the same thing, but they do something similar. By carefully observing and coding the facial expressions that accompany nice and nasty tastes, you can tell what an animal is enjoying and what they aren't.

Pleasure principles

This method is a breakthrough because it gives us another way of looking at how non-human species feel about things. Most animal psychology uses overt actions – things like pressing levers – as measures. So, for example, if you want to see how a reward affects a rat, you put it in a box with a lever and give it food each time it presses the level. Sure enough, the rat will learn to press the lever once it learns that this produces food. Taste reactivity creates an additional measure, allowing us insight into how much the animal enjoys the food, as well as what it makes it want to do.

From this, the neuroscientists have been able to show that wanting and liking are governed by separate circuits in the brain. The liking system is based in the subcortex, that part of our brain that is most similar to other species. Electrical stimulation here, in an area called the nucleus accumbans, is enough to cause pleasure. Sadly, you need brain surgery and implanted electrodes to experience this. But another way you can stimulate this bit of the brain is via the opioid chemical system, which is the brain messenger system directly affected by drugs like heroin. Like brain surgery, this is also NOT recommended.

Wanting happens in nearby, but distinct, circuits. These are more widely spread around the subcortex than the liking circuits, and use a different chemical messenger system, one based around a neurotransmitter called dopamine. Surprisingly, it is this circuit rather than the one for liking which seems to play a primary role in addiction. For addicts, a key aspect of their condition is the way in which people, situations and things associated with drug taking become reminders of the drug that are impossible to ignore. Berridge has hypothesised that this is due to a drug’s direct effects on the wanting system. For addicts any reminder of drug taking triggers a neural cascade, which culminates in feelings of desire, but crucially, without needing any actual enjoyment of the drug to occur.

The reason wanting and liking circuits are so near each other is that they normally work closely together, ensuring you want what you like. But in addiction, the theory goes, the circuits can become uncoupled, so that you get extreme wanting without a corresponding increase in pleasure. Matching this, addicts are notable for enjoying the thing they are addicted to less than non-addicts. This is the opposite of most activities, where people who do the most are also the ones who enjoy it the most. (Most activities except another Christmas tradition, watching television, where you see the same pattern as with drug addictions – people who watch the most enjoy it the least).

So now you know what do when you find yourself chomping your way through yet another packet of crisps over the holiday period. Watch your face and see if you are licking your lips. If you are, perhaps your liking circuits are fully engaged and you'll be happy with what you’ve eaten when you’re finished. If there's no lip-licking then perhaps your wanting circuits are in control and you need to exercise some self-restraint. Maybe after the next mouthful, though.

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